I was extremely interested to read your article on the listening mirrors in Kent (NCE 19 February).
Concrete Repairs Ltd (CRL) was commissioned by English Heritage as a specialist contractor on the project during 2003. These structures and their history are fascinating, and it is vital that they are preserved for future generations to be able to see and understand.
Major repairs were necessary to both the 61m parabolic wall and the 6m mirror to preserve some structural stability after local gravel extraction. CRL developed a repair strategy that not only reproduced the original surface textures and features, but also replicated the likely mix designs used in the original construction. An appreciation of the sensitivity of the mirrors was necessary in this planning.
For example, stitch drilling to fit new reinforcement was not possible because of the risk of major damage to the dishes.
Instead, new reinforcement was added by grouting bars into hand cut chases.
The contract was never intended to fully restore the mirrors but to prevent them from further deterioration until the future of the structures and their presentation and appearance has been decided by English Heritage.
Tony Rimoldi (M) managing director, Concrete Repairs, firstname.lastname@example.org